1. Field of the Invention
This invention relates generally to a battery charger of a storage battery pack. More specifically, it relates to a battery charger which detects the full charge of a storage batteries. By using a thermostat and a thermistor built into the storage battery pack.
2. Description of Related Art
A storage battery which can be used to power an electromotive tool is repeatedly used. It is known to rapidly charge such a battery by flowing a large current through it. Battery chargers for doing so are known. Such battery chargers detect a full charge of the storage battery by various methods. One type of detector uses a thermostat for detecting the full charge.
An example of a battery charger which uses the thermostat will now be explained with referring to FIG. 11 and FIG. 2. A storage battery pack 450 shown in FIG. 2 has a plural storage batteries B shown in FIG. 11, and a thermostat TH which opens at a predetermined temperature due to completion of charging of the storage batteries B, and a first, a second, a third input terminals t1, t2, t3, as shown in FIG. 11. battery charger 410 includes a direct current circuit 416 which rectifies commercially available AC power into direct current, a charging control circuit 412 that monitors the open/closed status of the thermostat TH, a constant voltage power supply 418 which supplies electric power to the charging control circuit 412, and a switch element 414 which stops the charge when the charging control circuit 412 detects that thermostat TH is open as a result of full charging of the storage batteries B.
A problem associated with the operation of this circuit is that when first input terminal t1 comes into contact with third input terminal t3 during a use, thermostat TH may break down and will no longer function properly. The circuit is subject to failure in another manner as well. When the storage battery pack 450 is used in a device which causes it to shake such as in an electromotive style tool, thermostat TH sometimes suffers a mechanical breakdown. When thermostat TH fails to operate properly, the charging completion of the storage batteries B is not detected, and the life time of the storage batteries B may became short due to improper charging.
It is accordingly an object of the present invention to provide a battery charger which can detect the completion of charging of a storage battery pack surely and accurately.
Further, an object of the present invention is to provide a battery charger which can charge a storage battery pack only under a low temperature.